1. Field of the Invention
This invention relates generally to a treatment process for anodic foils to be used in electrolytic capacitors. This invention also relates to a high capacitance, low leakage current anodic foil created by the foil treatment process, an electrolytic capacitor incorporating this anodic foil, and an implantable cardioverter defibrillator (ICD) incorporating this electrolytic capacitor.
2. Related Art
Compact, high voltage capacitors are utilized as energy storage reservoirs in many applications, including implantable medical devices. These capacitors are required to have a high energy density since it is desirable to minimize the overall size of the implanted device. This is particularly true of an implantable cardioverter defibrillator (ICD), also referred to as an implantable defibrillator, since the high voltage capacitors used to deliver the defibrillation pulse can occupy as much as one third of the ICD volume.
ICDs are typically implanted in patients suffering from potentially lethal cardiac arrhythmias. Arrhythmia, meaning "without rhythm," denotes any variance from normal cardiac rhythm. Heartbeat irregularities are fairly common and many are harmless. A severe heartbeat irregularity known as ventricular tachycardia refers to a runaway heartbeat.
Fibrillation is an irregular rhythm of the heart caused by continuous, rapid, electrical impulses being emitted/discharged at multiple locations known as foci in the heart's atria and ventricles. Because a fibrillating heart is unable to properly pump blood through a patient's body, the longer a patient is in fibrillation, the greater the potential damage that can occur to the patient's heart. Thus, after the start of fibrillation, it is preferable to apply defibrillating therapy to the patient as soon as possible. An ICD is designed to apply such therapy automatically and quickly to minimize damage to the heart.
An ICD monitors cardiac activity and decides whether electrical therapy is required. For example, if a tachycardia is detected, pacing or cardioversion therapy may be used to terminate the arrhythmia. If fibrillation is detected, defibrillation is the only effective therapy.
Both cardioversion and defibrillation require that a high voltage shock be delivered to the heart. Since it is impractical to maintain high voltage continuously ready for use, ICDs charge energy storage capacitors after detection of an arrhythmia and prior to delivering a shock to the heart.
An ICD system normally includes a high current electrical battery cell, an energy storage reservoir (i.e., charge capacitor), and a step-up transformer and power conversion circuitry to charge the capacitor. Typically, the ICD charges the charge capacitor to a high voltage (700-800 Volts).
Electrolytic capacitors are used in ICDs because they have the most nearly ideal properties in terms of size and ability to withstand relatively high voltage. Typically, these capacitors can be aluminum electrolytic capacitors (either rolled or flat).
Aluminum electrolytic capacitors having aluminum foil plates rolled into a very small volume are generally used in ICDs. By etching the surface of the aluminum foil, the surface area can be further increased such that the capacitance increases accordingly.
Since these capacitors must typically store approximately 30-40 joules, their size can be relatively large, and it is difficult to package them in a small implantable device. Currently available ICDs are relatively large (over 44 cubic centimeters (cc)), generally rectangular devices about 12-16 millimeters (mm) thick. A patient who has a device implanted may often be bothered by the presence of the large object in his or her pectoral region. Furthermore, the generally rectangular shape can in some instances lead to pocket erosion at the somewhat curved corners of the device. For the comfort of the patient, it would be desirable to be able to make smaller and more rounded ICDs. The size and configuration of the capacitors has been a major stumbling block in achieving this goal.
In ICDs, as in other applications where space is a critical design element, it is desirable to use capacitors with the greatest possible capacitance per unit volume. As mentioned above, one way to increase capacitance per unit area in a flat capacitor is to etch the surface of the anode foil perpendicular to the surface thereof. An ICD with flat geometry electrolytic capacitors is described in U.S. Pat. No. 5,131,388 to Pless et al. ("Pless"), which is incorporated herein by reference in its entirety. While such flat capacitors provide an improvement from a packaging and energy density standpoint, the energy or power density can still be greatly improved.
Conventionally, an electrolytic capacitor comprises a series combination of two or more capacitors, wherein each capacitor includes two electrodes (such as metal foils or plates that act as anodes) separated by an electrolyte (that acts as the cathode), with one or both metal foils having a thin dielectric film or barrier layer formed on their adjacent surfaces. Alternatively, the electrolytic capacitor can comprise a single, double, or a higher multiple number of metal anode plates having paper separators covering each anode layer and separating the anode layers from a cathode foil, such as those described in Pless.
Manufacturers of electrolytic capacitors of very small size (also referred to as microcapacitors) that support voltages of about 400 Volts or more face several difficulties. In particular, an important concern is how to prepare a metal foil (also referred to as an anodic foil which is used as an anode plate in the electrolytic capacitor) that maintains a high capacitance while at the same time has a reduced leakage current. The term "leakage current" refers to the current passing between an electrolyte and an anodic foil. Under conventional anodic foil preparation techniques, a barrier oxide layer can be formed onto one or both surfaces of a metal foil by placing the foil into an electrolyte bath and applying a positive voltage to the metal foil and a negative voltage to the electrolyte. This process (also referred to as electrolysis) oxidizes the surface of the metal foil. The oxide film formed during electrolysis normally has a thickness ranging from 0.006 to 1.0 micrometers (.mu.m). However, the oxide film should be sufficiently thick to support the intended use voltage. This oxide layer provides a high resistance to current passing between the electrolyte and the metal foils, also referred to as the leakage current. A high leakage current can result in the poor performance and reliability of an electrolytic capacitor. In particular, a high leakage current results in greater amount of charge leaking out of the capacitor once it has been charged.
One attempt to lower the leakage current of anodic foils is found in U.S. Pat. No. 5,449,448, issued to Kurihara et al. ("Kurihara"), which is incorporated herein by reference in its entirety. In order to improve the leakage current characteristics of the anode foil utilized in an electrolytic capacitor, Kurihara describes an aluminum foil treatment solution of organic acids or salts. Specifically, Kurihara describes a treatment solution that can include a straight chain saturated dicarboxylic acid with an odd number of carbons, a trans straight chain unsaturated dicarboxylic acid, or an organic acid having an aromatic ring and a carboxyl group. These organic acids are used as a "dip" following a hydration step, and prior to the application of a potential to form a barrier oxide layer. While this organic acid dip lowers the resulting leakage current of the barrier oxide layer formed, it is found in practice that this dip also lowers the capacitance of the metal foil by nearly 10 %, and is therefore impractical for commercial applications.